1. Field of the Invention
The present invention relates to a station-side apparatus and a passive optical network (PON) system using the apparatus.
2. Description of the Related Art
PON systems have been conventionally known for subscriber optical fiber network systems provided for subscribers' homes including ordinary households (see Patent Document 1, for example). A PON system comprises a station-side apparatus {hereinafter, also referred to as an optical line terminal (OLT)}, multiple subscriber-side apparatuses {hereinafter, also referred to as optical network units (ONUs)}, an optical fiber provided between the OLT and ONUs, and an optical splitter that branches the optical fiber. In a PON system, a single optical fiber connected to an OLT is shared by multiple ONUs, thereby constructing an optical communication network at low cost.
As a PON system, GE-PON {Gigabit Ethernet (registered trademark)-Passive Optical Network}, which is standardized in IEEE802.3ah and offers a transmission rate of 1 Gbps, is well known.
In recent years, 10G-EPON {10 Gigabit-Ethernet (registered trademark) Passive Optical Network}, which offers a transmission rate of 10 Gbps, has been standardized in IEEE802.3av as a next-generation technology of GE-PON. A 10G-EPON provides a higher transmission rate of 10 Gbps compared to a GE-PON but basically adheres to the techniques of GE-PON. For upstream communication from an ONU to an OLT, 10G-EPON employs a burst transmission technique using control by means of the dynamic bandwidth allocating function {hereinafter, also referred to as dynamic bandwidth allocation (DBA)}.
An upstream burst signal standardized in IEEE802.3ah or IEEE802.3av comprises a burst header at the beginning, which is used for bit synchronization and frame synchronization, and a data frame subsequent thereto. In a GE-PON, a burst header contains repetition of an idle pattern defined using 8B/10B encoding and is used for stabilization of an optical receiver that receives burst signals and for establishment of burst signal synchronization. In a 10G-EPON, on the other hand, a burst header consists of a sync pattern and a burst delimiter. The sync pattern is repetition of a 66-bit fixed pattern and is used for stabilization of an optical receiver that receives burst signals. The burst delimiter is a 66-bit fixed pattern, which is different from that in the sync pattern, and is used for establishment of burst signal synchronization.
When an OLT receives an upstream burst signal from an ONU, an optical receiver in the OLT performs processes including photoelectric conversion, clock extraction, and regenerating. Then, serial-parallel conversion is performed on the burst signal, and an idle pattern is detected in a GE-PON and a burst delimiter is detected in a 10G-EPON, so as to establish burst signal synchronization.
In a GE-PON, when the data pattern in a burst header has perfectly matched an idle pattern defined using 8B/10B encoding, it is determined that a burst signal has been detected, so that synchronization of the burst signal is established. In a 10G-EPON, on the other hand, when 55 bits or more in a 66-bit burst delimiter have matched corresponding bits in a fixed pattern, it is determined that a burst delimiter has been detected, so that synchronization of the burst signal is established. In a 10G-EPON, error correction using forward error correction (FEC) is required, and such correction is performed after the detection of a burst delimiter. Accordingly, at the time of detection of a burst delimiter, the burst signal may possibly have a bit error rate of about 10×10−3. Therefore, if the detection of a burst delimiter is performed under the same conditions as in a GE-PON, it might be unable to achieve burst signal synchronization appropriately. In consideration thereof, eased detection conditions are set for a burst delimiter in a 10G-EPON when compared to those in a GE-PON, enabling appropriate establishment of burst signal synchronization.    [Patent Document 1] Japanese Patent Application Laid-open No. 2007-173908
However, depending on its specifications, an optical receiver for an OLT, which receives burst signals, may output a noise signal during a period between times when the optical receiver receives burst signals, i.e., a period in which no optical signals are input. If such a noise signal happens to meet the detection conditions for a burst delimiter, false synchronization of a burst signal might be caused.